Button1_Click is not a member of ASP default2_aspx

These type of error are resolved by -

• Make sure that youre namespaces, classes and events are correctly named. It happens when you rename the name of the class or file and forget to also rename the class or vice versa.
• Those are event handlers that don't exist in your code-behind file. You have to add them, and then it should be good to go.